Direct Observation of Carrier Trapping Processes on Fe Impurities in mc-Si Solar Cells

Abstract

n-situ Mössbauer studies on 57Fe solute atoms in Si solar cells are performed: (1) GeV-57Mn/57Fe implantation into Si solar cells, (2) 57Fe diffused n-type Si under light illumination; (3) 57Fe diffused solar cells under applying external voltages. The carrier trapping cross sections for the interstitial components with different charge states, Fei+ and Fei2+, can be successfully obtained by evaluating the dynamical charge fluctuations within a time scale of 100ns between Fei+ and Fei2+ which appear in the Mössbauer spectra of 57Fe doped mc-Si solar cells. We further measure the distributions of Fei+ and Fei2+ by a Mössbauer Microscope, which we have been developing. The present results provide us a possibility to clarify the carrier trapping process on an atomistic scale directly on the Fe impurities in Si-solar cells.

Mössbauer Spectroscopy on Fe Impurities in Si Materials

“…Nowadays, one could use much better theoretical tools [23], but the isomer shifts deduced for the charged interstitial and substitutional Fe atoms do not agree with the experimental values. Our assignments for the charged Fe states are, therefore, proposed by taking into account of the experimental conditions at the 57 Fe nuclear probes in Si materials [17][18][19][20], referring to the theoretical predictions for the Fe deep levels in Si [24]. It should be mentioned that all the spectra did not contain any Fe-silicides and Fe-oxides whose isomer shifts are well known.…”

“…The isomer shift, which is proportional to the electron density at the nucleus, corresponds to the energy shift of the nuclear level for a Mössbauer component on a Doppler velocity scale of mm/s against the spectral centre of α-Fe at room temperature. The data have been obtained in different Mössbauer experiments on 57 Fe impurities, which were differently introduced into Si samples, and subsequently measured at different temperatures: (1) deposition at room temperature, and diffusion and measurements at high temperatures up to 1273K [11,14,15], (2) highly energetic implantation of the mother isotope of 57 Mn/ 57 Fe into Si [12,13,17], and (3) deposition and diffusion, and measurements at room temperature [16,[18][19][20]. All the results can be analysed by the new model based on the interstitial Fe with neutral, +1, and + 2 states (defect associated), in addition to substitutional Fe with neutral, possibly +1 and -1 states.…”

“…Mössbauer spectroscopy appears to be ideal to characterize Fe impurities in Si crystal [6][7][8][9][10]. Recently, we have performed a series of the experimental investigations [10][11][12][13][14][15][16][17][18][19][20] using Mössbauer spectroscopy on Fe impurities in p-type and n-type Si materials such as single crystal Si, multi-crystal line (mc) Si wafers, and even mc-Si solar cells, the results are appealing that we need a new picture for Fe impurities in Si materials: the Fe impurities in the Si matrix exist not only on the interstitial sites with Fe int 0/+ and Fe int + -Bpair, but also on a higher charge state of Fe int 2+ associated with defects as well as on the substitutional sites with Fe sub 0 and Fe sub -. Furthermore, these components are found to transform each other in the Mössbauer spectra by changing experimental conditions such as temperature, external voltage, electron or light irradiation, and external stress as well as by changing the Fermi level, carriers and their concentrations, and the device structures in the vicinity of 57 Fe probes.…”